The great rope in the sky

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Scandinavians of old thought northern lights were the reflection of giant schools of herring in the sea. Gone are those days: physicists now know that charged particles emitted by the Sun the solar wind--interact with Earth's upper atmosphere near the poles to generate auroras. And satellites recently revealed the peculiar avenues the particles sometimes travel: gigantic electro magnetic structures called magnetic ropes.

The ropes are twisted bundles of magnetic fields temporarily generated when unusually strong solar winds buffet Earth's own magnetic field, 40,000 miles above the ground. Magnetic ropes stretch from the Sun into Earth's upper atmosphere and serve as conduits for solar-wind energy. But all that was just so much theory until last May 20th. That's when five NASA satellites mapped the first confirmed magnetic rope, as wide as Earth itself. Vassilis Angelopoulos and David Sibeck, two NASA scientists, say the rope formed for only a few minutes, but it conducted a massive amount of energy earthward that powered intense auroras.

NASA had launched the satellites several months earlier, as part of a mission called THEMIS, specifically to study such magnetic disturbances, which not only provoke spectacular sky shows, but can also fry satellites and mess up radio communication. (Presented at the American Geophysical Union's fall meeting)

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